Summary Around 12% of women in the United States have problems getting pregnant and carrying a baby to term, with 7% of women and their partners being diagnosed as infertile. Some diagnoses of infertility can be overcome through Assisted Reproductive Technologies (ART), however these technologies require each partner to make functional gametes (eggs or sperm) for success. For those individuals incapable of making gametes, ART is not an option for overcoming a diagnosis of infertility. In this grant, we are developing a model for fertility restoration to the infertile patient using induced pluripotent stem cells (iPSCs). This basic research proposal is aimed at testing the hypothesis that prenatal and neonatal gonadal somatic cells are required to instruct male and female germline cell differentiation using non human primate (NHP) iPSCs. The proposal under consideration is based upon the scientific premise with mouse (m) models that male and female miPSCs differentiated into mouse primordial germ cell (PGC)-like cells (mPGCLCs) will undergo sex-specific differentiation when transplanted with prenatal gonadal somatic cells, or directly into neonatal niches. The success of this technology in the mouse was first built upon a fundamental understanding of mouse germline cell development, particularly during prenatal life. Here, we propose three aims in which we will use developmental biology, genomics and transplantation to understand the cell and molecular basis of sex- specific PGC differentiation in vivo using NHPs. This knowledge will be used to differentiate male and female NHP iPSCs towards PGCLCs that can undergo sex-specific differentiation using the signaling logic of the prenatal gonad, or alternatively following transplantation or culture with prenatal or neonatal gonadal somatic cells. At the conclusion of this proposal, we will have determined the extent to which NHP PGCLCs are capable of sex-specific differentiation within prenatal and neonatal gonadal niches. This work will be used to inform future studies aimed at recreating male and female gametogenesis from NHP's entirely in vitro.